Fermilab experiment discovers a heavy relative of the neutron:
photos and graphics

Once produced, the neutral Xi-sub-b () particle travels about a millimeter before it disintegrates into two particles: the short-lived, positively charged Xi-sub-c () and a long-lived, negative pion (π-). The Xi-sub-c then promptly decays into a pair of long-lived pions and a Xi particle (), which lives long enough to leave a track in the silicon vertex system (SVX) of the CDF detector before it decays a pion and a Lambda (Λ). The Lambda particle, which has no electric charge, can travel several centimeters before decaying into a proton (p) and a pion (π). Credit: CDF collaboration.

Baryons are particles made of three quarks. The quark model predicts the baryon combinations that exist with either spin J=1/2 (this graphic) or spin J=3/2 (not shown). The graphic shows the various three-quark combinations with J=1/2 that are possible using the three lightest quarks--up, down and strange--and the bottom quark. The CDF collaboration announced the discovery of the neutral Xi-sub-b (), highlighted in this graphic. Experiments at Fermilab’s Tevatron collider have discovered all of the observed baryons with one bottom quark except the Lambda-sub-b, which was discovered at CERN. There exist additional baryons involving the charm quark, which are not shown in this graphic. The top quark, discovered at Fermilab in 1995, is too short-lived to become part of a baryon.

The CDF collaboration has observed 25 Xi-sub-b candidates in their data. The analysis established the discovery of the neutral Xi-sub-b baryon at a level of 7 sigma. Scientists consider 5 sigma the threshold for discoveries. CDF scientists measured the mass of the neutral Xi-sub-b to be 5.7878 GeV/c2. Credit: CDF collaboration.

The Fermilab accelerator complex accelerates protons and antiprotons close to the speed of light. Converting energy into mass, the Tevatron collider can produce particles that are heavier than the protons and antiprotons that are colliding. The Tevatron produces millions of proton-antiproton collisions per second, maximizing the chance for discovery. Two experiments, CDF and DZero, search for new types of particles emerging from the collisions.

The CDF detector, about the size of a 3-story house, weighs about 6,000 tons. Its subsystems record the "debris" emerging from high-energy proton-antiproton collisions. The detector surrounds the collision point and records the path, energy and charge of the particles emerging from the collisions. This information can be used to find and determine the properties of the Xi-sub-b particle.

Six quarks--up, down, strange, charm, bottom and top--are the building blocks of matter. Protons and neutrons are made of up and down quarks, held together by the strong nuclear force. The CDF experiment now has observed the neutral Xi-sub-b particle, which contains an up (u), strange (s) and bottom quark (b).